An alternator outputs current based on demand to maintain adequate system voltage. This is why your lights might dim when you suddenly increase current demand (turn AC on full blast, etc)...there is a bit of a lag between when the current is demanded and when the alternator can supply the additional current. Besides starting the car, this is what the battery is for...to supply the additional current needed during these sudden increases before the alt picks up the slack.Think about it logically....if there was only 100A of demand but the alt was producing 250A....where would the additional 150A of current "go"? Think of it this way.....current isn't just produced, it is drawn by the load. There has to be a demand for the current, which is based on the load and voltage. Simple test. Get a clamp meter and put it on your alt cable. Start the car with all accessories off and then gradually start adding loads by turning on accessories. agreed